Highlight specific edges of a shape
You can select the vertices you want and duplicate them.  Right click to send them back to their original position.  Then press P and select Separate by selection.

Go back to object mode and select your new object.
If your selection has any faces, enter edit mode and delete only faces.

At this point you have a circle, or a loop, or maybe just a line of vertices.  You're now able to convert this to a curve from Object mode.

You can give this some bevel depth.

And then apply a shader.  For example, volumetric emission.